It is known in the state-of-the-art to measure the flow velocity or the mass or volumetric flow rate of a liquid and/or gaseous medium by means of a flow sensor on which a heat source and a suitable temperature sensing means are arranged. The flow leads to a change in the temperature distribution generated by the thermal source which can be measured using the temperature sensing means.
The detection of gas (air) bubbles in a liquid flow is of interest e.g. in a medical infusion because air supplied to a patient's blood vessels can be harmful or even lethal. Nowadays a special sensor in infusion pumps is used to detect air in the line.
In traditional thermal flow sensors a small amount of heat is generated by a heat source and the liquid flow rate is deduced from a temperature sensor measuring the cooling effect of the flowing liquid. The observed cooling down is the same for both flow directions (forwards and backwards), leading to a lack of sensitivity at low flow rates in the vicinity of a vanishing flow rate. This problem can be remedied by using a differential measuring procedure involving two temperature sensors, wherein one temperature sensor is arranged downstream the heat source and the other one is arranged upstream the heat source This measuring technique also has the advantage of receiving a faster signal response as well as a reduced dependency on the temperature of the liquid.
However, when such a differential signal is used, the presence of a gas or air bubble in the sensor cannot be distinguished in a satisfying manner from a no-flow condition where the line is filled with liquid, but the liquid stands still (zero flow rate).